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BSA Calculator

Body surface area (BSA) represents the total external skin area of the human body, expressed in square meters. Clinicians widely use BSA calculations for determining chemotherapy doses, adjusting medication regimens, and estimating metabolic rates more reliably than weight alone. Whether you're a healthcare provider, researcher, or simply curious about your physiology, this calculator computes BSA from your height and weight using validated formulas.

Last updated: April 22, 2026

Creators Małgorzata Koperska, MD

Reviewers Łucja Zaborowska and Adena Benn

514 people find this calculator helpful

Understanding Body Surface Area

Body surface area differs fundamentally from simple weight measurements because it accounts for how your body distributes across three dimensions. A person weighing 70 kg with a height of 170 cm will have a BSA around 1.83 m², which serves as a reference point for standardized clinical calculations.

Healthcare professionals prefer BSA over body weight for several reasons:

Drug metabolism: Many chemotherapy agents and immunosuppressants require BSA-based dosing to minimize toxicity.
Renal function assessment: Glomerular filtration rate (GFR) calculations are normalized to a standard BSA of 1.73 m² to enable meaningful comparisons across patients.
Metabolic accuracy: BSA correlates more closely with energy expenditure and organ function than weight alone, particularly in obese individuals.

Average BSA values vary by age and sex, with infants typically measuring 0.25 m² and healthy adult males averaging 1.9 m².

BSA Calculation Methods

Several validated formulas exist for estimating BSA, each derived from cadaver studies or clinical populations. The Du Bois formula, developed in 1916, remains the most widely adopted in clinical practice due to its simplicity and reliability. Other established methods include Mosteller, Haycock, Gehan & George, and Fujimoto formulas, which yield slightly different results depending on the population they were calibrated against.

Du Bois: BSA = 0.007184 × weight^0.425 × height^0.725

Mosteller: BSA = √(weight (kg) × height (cm) ÷ 3600)

Haycock: BSA = 0.024265 × weight^0.5378 × height^0.3964

Gehan & George: BSA = 0.0235 × weight^0.51456 × height^0.42246

Fujimoto: BSA = 0.008883 × weight^0.444 × height^0.663

weight — Body weight in kilograms (kg)
height — Height in centimeters (cm)
BSA — Body surface area in square meters (m²)

When and Why BSA Matters

BSA-based dosing is particularly critical in oncology, where chemotherapy toxicity margins are narrow. A 10% error in drug dosing can mean the difference between therapeutic benefit and severe harm. Adult BSA typically ranges from 1.5 to 2.0 m², with larger individuals reaching 2.5 m² or more.

Pediatric populations require special attention because children's BSA scales non-linearly with weight. A 5-year-old child weighing 20 kg has a BSA around 0.8 m², substantially lower than a proportionally scaled down adult. Using weight-only dosing in children risks underdosing or overdosing medications.

Beyond medication, BSA appears in formulae for:

Estimating basal metabolic rate and caloric requirements in critical care.
Normalizing cardiac output and other physiological measurements.
Assessing body composition changes in obesity or malnutrition studies.

Converting Between Imperial and Metric

If your measurements are in pounds and inches, conversion is straightforward:

Weight conversion: Divide pounds by 2.2046 to get kilograms. For example, 180 lbs ÷ 2.2046 = 81.65 kg.
Height conversion: Convert feet and inches to total inches, then multiply by 2.54 to get centimeters. A height of 5 ft 10 in equals 70 inches; 70 × 2.54 = 177.8 cm.

Alternatively, use the modified Mosteller formula designed for imperial units: BSA = √(weight (lbs) × height (inches) ÷ 3.131). This formula avoids intermediate conversions while yielding identical results to the metric version.

Common Pitfalls and Considerations

Accurate BSA calculation requires attention to measurement technique and formula selection.

Formula selection affects results — Different formulas produce variations of 5–10% depending on the patient's build. Du Bois and Mosteller generally agree closely, but Haycock can yield notably higher values in children. Always verify which formula your clinical setting requires; some institutions mandate specific equations for regulatory compliance.
Measurement accuracy is critical — BSA calculations amplify small measurement errors because height is raised to powers between 0.4 and 0.7. A 2 cm error in height changes BSA by roughly 1–2%. Use calibrated scales and measuring devices; avoid rounding intermediate values when performing manual calculations.
BSA is an approximation, not a law — While BSA-based dosing is standard, it assumes your metabolism matches population averages. Severely obese, cachectic, or edematous patients may not fit the model well. Some oncologists adjust doses based on actual body weight rather than BSA alone, particularly in elderly or fragile populations.
Reference values apply to healthy adults — The famous 1.73 m² figure represents a 70 kg average adult. Infants, very tall athletes, and individuals with extreme body compositions will differ substantially. Always contextualize results within age- and sex-specific norms rather than treating 1.73 m² as a universal standard.

Frequently Asked Questions

What is a normal BSA for adults?

Most healthy adults fall within 1.5–2.0 m². The reference standard of 1.73 m² corresponds to a 70 kg person of average height (approximately 170 cm). Sex-based differences are modest; adult males average around 1.9 m² while females average 1.6 m², primarily due to differences in height and weight distributions. Extremely tall or obese individuals can exceed 2.5 m², whereas petite adults may measure below 1.4 m².

Why do doctors use BSA instead of body weight for medication dosing?

Body surface area correlates more accurately with organ function, metabolic rate, and drug clearance than weight alone. This relationship is especially important for chemotherapy, where narrow therapeutic windows demand precision. A 100 kg person with high muscle mass differs physiologically from a 100 kg person with high body fat; BSA accounts for this variation. Additionally, pediatric dosing requires BSA-based calculations because children's proportions differ dramatically from scaled-down adults, making weight-based approaches unreliable.

How should I measure height and weight for accurate BSA calculation?

Measure weight using a calibrated digital scale with the person in light clothing and no shoes; record to the nearest 0.1 kg. Height should be measured using a stadiometer with the person standing barefoot, heels against a wall, and eyes facing forward. Both measurements should be repeated, and averages used if significant variance appears. Even small errors—particularly in height—substantially affect BSA estimates since height exponents range from 0.4 to 0.7 across different formulas, magnifying measurement mistakes.

Which BSA formula should I use?

The Du Bois formula remains the clinical standard and is suitable for adults and older children. Mosteller offers a mathematically simpler alternative that yields nearly identical results. For pediatric patients under 10 years, Haycock formula often provides better accuracy. Gehan & George and Fujimoto are less commonly used but may be specified by individual protocols. When in doubt, verify your healthcare institution's standard formula to ensure consistency with institutional dosing guidelines.

Is BSA valid for obese or very tall patients?

BSA formulas were derived from populations with relatively normal body compositions. Severely obese patients may have lower metabolic rates than their BSA suggests, potentially leading to dose adjustments. Very tall individuals generally follow BSA predictions reliably. Some clinicians cap BSA calculations for obese patients using adjusted weight formulas rather than actual weight to prevent overdosing. Always discuss dosing concerns with your pharmacist or oncologist, as individual circumstances may warrant modifications to standard BSA-based regimens.

What does the 1.73 m² reference standard represent?

The 1.73 m² figure is the average BSA of a healthy 70 kg adult and serves as the normalization constant in many clinical formulas, particularly for glomerular filtration rate (GFR) estimation. This standardization allows meaningful comparison of organ function across patients of different sizes. However, 1.73 m² should not be treated as an ideal or target BSA; it is simply a reference point for mathematical standardization. Your personal BSA depends entirely on your height and weight distribution and will vary naturally across the population.

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